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Journal of Alloys and Compounds 603 (2014) 7–13
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Hydrogenation properties and crystal structure of YMgT4 (T = Co, Ni, Cu)
compounds
V.V. Shtender a, R.V. Denys a,b, V. Paul-Boncour c, A.B. Riabov a, I.Yu. Zavaliy a,⇑
a
Karpenko Physico-Mechanical Institute, NAS of Ukraine, 5 Naukova St., 79601 Lviv, Ukraine
Hystorsys AS, P.O. Box 45, Kjeller NO-2027, Norway
c
Institut de Chimie et des Matériaux de Paris Est, CMTR, CNRS and U-PEC, 2-8 rue H. Dunant, 94320 Thiais, France
b
a r t i c l e
i n f o
Article history:
Received 20 February 2014
Received in revised form 6 March 2014
Accepted 7 March 2014
Available online 17 March 2014
Keywords:
Hydrogen storage
Metal hydride
Rare Earth compounds
Magnesium compounds
Pressure–composition–temperature
relationships
Crystal structure
a b s t r a c t
New two ternary YMgCo4 and YMgCu4 and one quaternary YMgCo2Ni2 compounds have been synthesized by mechanical alloying with further annealing. The hydrogenation capacity of YMgCo4 reaches
6.8 at. H/f.u. The Pressure-Composition-Temperature studies of YMgCo4–H2 and YMgNi4–H2 systems
revealed that introduction of magnesium, accompanied by shrinking of the unit cell, decreases the
stability of hydrides comparing to binary YCo2 and YNi2 compounds. The values of heat and entropy
of the YMgCo4H6.8 hydride formation were calculated: DH = 27.9 ± 0.8 kJ mol–1 H2 and DS =
93.4 ± 2.6 J mol1 H2 K1. The YMgCo2Ni2–H2 system shows intermediate thermodynamic properties
compared to the ternary hydrides (DH = 28.8 ± 0.2 kJ mol–1 H2 and DS = 117.6 ± 2.4 J mol–1 H2 K1).
The YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compounds with a
cell volume expansion of 23 and 14.4% respectively. It is shown that the YMgCu4 compound does not
interact with hydrogen under normal conditions.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
Recently substantial efforts of researchers have been focused on
the development of magnesium and magnesium-based materials
for hydrogen storage technologies. There are two main directions
of such studies. The first one is the creation of nanoscale Mg-based
composites, which are capable of absorbing and desorbing hydrogen at substantially lower temperatures than pure magnesium metal [1,2]. Another direction is the development of new ternary or
pseudo-binary Mg-containing compounds, able to reversibly absorb hydrogen from the gas phase or electrochemically. Among
such compounds one should mention recently studied Mg3TNi2
(T – Ti, Al, Mn) derivatives of Ti2Ni-type [3–5] and (R1xMgx)nNim
intermetallic compounds (R = La, Ce or other rare earth metals)
[6–8].
The hybrid R1xMgxNi3 or (R1xMgx)2Ni7 compounds have great
potential for use as the new generation negative electrodes of Ni–
MH batteries. Their electrochemical discharge capacity is close to
400 mAh/g [7], which is 25% higher than the capacity of industrial
LaNi5-based electrodes. The RMgNi4 compounds with a cubic
MgCu4Sn structure type belong to the promising hydrogen absorbing materials also. For example, the reversible capacity (1.05 wt.%
⇑ Corresponding author. Tel.: +380 50 9833506.
E-mail address: [email protected] (I.Yu. Zavaliy).
http://dx.doi.org/10.1016/j.jallcom.2014.03.030
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
H) and the hydride formation heat (35.8 kJ/mol H2) of the YMgNi4
compound [9] are close to that for the LaNi5–H2 system [10], which
makes this material attractive for hydrogen storage.
RMgNi4 ternary intermetallics (where R – rare earth metal) are
crystallized in the MgCu4Sn structure type, which is an ordered
variant of the AuBe5 (F–43m) type, where Mg occupies the position
of Au (4a) and Sn one of the Be (4c) sites (Fig. 1). They can be considered also as derivative for the structure of cubic Laves phase
(MgCu2 type, Fd–3m) [11]. For the first time CeMgNi4 ternary compound was described in [12]. After this Aono et al. [9] synthesized
YMgNi4 compound and studied its hydrogen absorption properties.
RMgNi4 isostructural compounds were synthesized for a number of
other rare-earth metals [13,14].
Interaction of the RMgNi4 compounds with hydrogen was studied for R = Y [9,15] and R = La, Nd [16]. They absorb the same
amount of hydrogen (H/M = 0.60.67). Contrary to RNi2 hydrides
[17,18] these compounds are stable against hydrogen-induced
amorphisation and disproportionation at room temperature and
1–2 MPa H2 pressure. Recently we have shown that the isostructural compounds are formed also in R–Mg–Co systems [19]. The
CeMgCo4 compound was synthesized and its hydrogen absorption
properties were studied. This compound readily absorbs hydrogen
up to 1.0 H/M at room temperature and 10 MPa H2 pressure [19],
whereas under the same conditions isostructural CeMgNi4 compound is unable to form intermetallic hydride [20].
8
V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13
Fig. 1. Comparison of the structure of RNi2 and RMgNi4 compounds. Upon the transition from RNi2 to RMgNi4 (Fd–3m ? F–43m) the site 8a (0 0 0), filled by atoms R in RNi2
Laves phases, splits into two sites: 4a (0 0 0) – R and 4c (1/4 1/4 1/4) – Mg.
2. Experimental part
Cu-Kα
Intensity (a.u.)
Yobs
Ycalc
Yobs - Ycalc
Bragg position
YMgCo2Ni2
Y2O3
20
30
40
50
60
70
80
90
2θ (deg.)
Fig. 2. X-ray diffraction pattern of YMgCo2Ni2 alloy. Observed (Yobs), calculated
(Ycalc), difference (YobsYcalc) diffraction profiles and Bragg’s peaks positions for
YMgCo2Ni2 (97.1 wt.%) and Y2O3 (2.9 wt.%) phases are shown.
Cu-Kα
Intensity (a.u.)
Yobs
Ycalc
Yobs - Ycalc
Bragg position
As starting materials for preparation of the RMgT4 compounds we used RT4 alloy precursors and Mg powder (Alfa Aesar, 325 mesh, 99.8%). RT4 alloys were prepared by arc melting from pure metals (purity P99.9%) in an atmosphere of
purified argon. Alloys were crushed and mixed with magnesium powder in the proportions corresponding to RMgT4 stochiometry. The mixtures were ball-milled under Ar atmosphere in sealed stainless steel vials using SPEX 8000D mill for 4–8 h.
After the grinding the powder alloys were annealed under argon in tantalum container, which was placed in a sealed stainless steel autoclave, at 600–800 °C for
8 h. Afterwards, the alloys were quenched to room temperature.
The hydrogenation properties were measured with a Sievert type apparatus in
order to obtain the Pressure–Composition–Temperature (PCT) diagrams as well as
saturated hydride. The samples were activated by heating under vacuum.
Phase analysis of the samples was carried out by X-ray powder diffraction
(XRD) (DRON-3.0 diffractometer and Brucker D8) with Cu Ka radiation for Ni/Cucontaining alloy and Fe Ka radiation for RMgCo4 alloy. Crystal structures of the
compounds were refined by the Rietveld method from the diffraction data using
Fullprof software [21].
In situ Synchrotron Radiation (SR) XRD study was carried out on the BM01B line
at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) on highresolution diffractometer using monochromatic X-ray beam (k = 0.5012 Å). Thin
quartz capillary (diameter 0.5 mm, wall thickness 0.01 mm) was filled by the investigated alloy powder (2–5 mg) and placed in a special quartz cell mounted on a
goniometer head and connected to the gas system through a flexible plastic tube.
The heating and cooling of the sample was performed using a programmable cryofluidic system with working temperature range 77–500 K. Initially the sample was
heated in a dynamic vacuum to 420 K and when one reaches this temperature the
hydrogen gas (99.999% purity) was introduced in the cell. Under these conditions,
‘‘activation’’ of the alloy was done and a solid solution of hydrogen in Intermetallic
Compounds (IMC) was formed. Further, the sample was slowly cooled (5 K/min) to
room temperature to achieve the hydride formation.
3. Results and discussions
3.1. Synthesis of YMgT4 compounds (T = Co, Ni, Cu)
20
30
40
50
60
70
80
90
2θ (deg.)
Fig. 3. X-ray diffraction pattern of YMgCu4 alloy. Observed (Yobs), calculated (Ycalc),
difference (Yobs–Ycalc) diffraction profiles and Bragg’s peaks positions are shown.
In the present work, the existence of YMgCo4 and YMgCu4 compounds as well as YMgCo2Ni2 intermediate compound from the Y–
Mg–Ni–Co system has been shown and their structural properties
analyzed. Their hydrogen absorption-desorption properties were
investigated. The crystal structure of the YMgCo4, YMgNi4 and
YMgCo2Ni2 hydrides has been determined.
In all cases, practically single phase samples were synthesized
for the RMgT4 compounds. XRD patterns of the YMgCo2Ni2 and
YMgCu4 samples were refined and shown in Figs. 2 and 3, respectively. XRD patterns of the YMgCo4 and YMgNi4 samples were used
for comparison with those of the corresponding hydrides (see
Figs. 8a and 9a). Crystallographic parameters of YMgT4 compounds
obtained from X-ray diffraction data are given in Table 1.
Lattice parameters obtained in this work for YMgNi4 compound
(a = 7.013(3) Å) are in good agreement with literature data
(a = 7.01 Å) [9]. New compounds based on cobalt and copper are
characterized by slightly larger lattice parameter than the Ni-containing analogues, because of larger atoms (rNi = 1.246 Å,
rCo = 1.252 Å, rCu = 1.278 Å). As it was shown for RMgNi4 compounds [14], disordering of Mg and R atoms, caused by the different conditions of synthesis, leads to the increase of unit cell
parameters. Positional disordering is also observed in the case of
YMgCo4 (Table 1). In all other cases, the refinement showed com-
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V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13
pletely ordered structures with 100% occupancy of site 4c by Mg
and site 4a by R atoms.
Table 1
Crystallographic parameters of YMgT4 compounds (T = Co, Ni, Cu) with the MgCu4Sn
structure type (space group F–43m).
Compounds
YMgNi4
YMgCo2Ni2
YMgCo4
YMgCu4
Cell parameters
a (Å)
V (Å3)
7.0129(3)
344.90(3)
7.0247(1)
346.64(1)
7.0596(3)
351.84(3)
7.2301(2)
377.96(1)
Atomic positions
Y in 4a (0 0 0)
nY/nMga
Biso, (Å2)
1.0(-)
0.63(5)
1.0(-)
0.57(4)
0.87(1)/0.13(1)
0.24(5)
1.0(–)
1.4(5)
1.0(–)
0.5(1)
0.5Ni + 0.5Co
0.6249(2)
0.43(4)
0.89(1)/0.11(1)
1.1(1)
Co
0.6249(5)
0.37(3)
1.0(–)
1.2(1)
Cu
0.6241(1)
1.59(3)
Mg in 4c (1/4, 1/4,
nMg/nYa
Biso, (Å2)
T in 16e (x, x, x)
x
Biso (Å2)
a
1/4)
1.0(–)
0.6(1)
Ni
0.6243(2)
0.44(3)
3.2. Hydrogen absorption properties of YMgT4 (T = Co, Ni, Cu) alloys
In this work we investigated the hydrogen absorption properties of new YMgCo4 and YMgCu4 compounds. Hydrogen absorption
properties of the YMgNi4 compound were described in literature
[9,15]. The hydrogenation properties of Ni-, Co- and Cu-based compounds were studied and the structure of their corresponding hydrides compared.
3.2.1. Hydrogen absorption properties of the YMgNi4 compound
Hydrogen absorption capacity of YMgNi4 compound (MgCu4Sntype) at 313 K and 4 MPa hydrogen pressure was equal to
1.05 wt.%. A change in the structure upon the formation of intermetallic hydride has been reported [9], however, the structure of
this hydride has not been determined so far. In this work the
hydrogen absorption capacity of YMgNi4 compound in first hydrogenated cycle reached 3.96 at. H/f.u. (1.13 wt.%) at room temperature and 2 MPa H2 pressure. This value is in good agreement with
literature data [9,15]. The curves of the first hydrogenation of
YMgCo4, YMgNi2Co2 and YMgNi4 alloys are shown in Fig. 4.
After several cycles of full absorption and desorption PCT isotherms were measured at temperatures 273, 293, 313 and 333 K
in H2 pressure range from 0.01 to 2.5 MPa. Absorption and desorption of hydrogen occurs with a single plateau, which corresponds
to the a-YMgNi4H0.5 M b-YMgNi4H4 transition and reversible
capacity is equal to 1 wt.% H. At room temperature the equilibrium
pressure is 0.54 MPa for absorption and 0.17 MPa for desorption.
Desorption isotherms for the YMgNi4–H2 system were measured
and van’t Hoff plots have been built (Fig. 5). Note that the equilibrium pressure of hydrogen desorption obtained in our work is in
very good agreement with the results of [9], see Table. 2. However,
the values of heat and entropy hydride formation significantly
differ: DH = –33.1 ± 0.7 kJ mol1 H2 and DS = –117.6 ± 2.4 J mol1
H2 K1, compared with DH = 35.8 ± 0.4 kJ mol1 H2 and
Mixed filling of the positions of R and Mg atoms, nY + nMg = 1.
Fig. 4. First hydrogenation curves of YMgNi4, YMgCo2Ni2 and YMgCo4 compounds.
CH (wt.% H)
0.0
0.5
1.0
10
10
(a)
333 K
(b)
313 K
293 K
273 K
1
2
2
PH (MPa)
PH (MPa)
1
0.1
0.1
0.01
0.01
0
1
2
CH (H at./f.u.)
3
4
3.0
3.2
3.4
-1
1000/T (K )
Fig. 5. PCT desorption isotherms (a) and van’t Hoff plot (b) for the YMgNi4–H2 system.
3.6
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V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13
formation of an intermediate hydride phase was observed previously for the CeMgCo4 compound. At absorption–desorption isotherms for this compound there are two distinct plateaus, which
correspond to the formation of b-CeMgCo4H4 and c-CeMgCo4H6
hydride phases [19]. Therefore, the interaction in the YMgCo4–H2
system has been studied in more detail, in particular by in situ Xray diffraction.
Table 2
Comparison of desorption equilibrium pressure and thermodynamic parameters for
YMgNi4–H2, YMgCo2Ni2–H2 and YMgCo4–H2 systems.
T (K)
273
293
313
333
353
DHdes (kJ mol–1 H2)
DSdes J mol1 H2 K1
YMgNi4–H2
YMgCo2Ni2–H2 YMgCo4–H2
Pdes (MPa)
0.068
0.17
0.42 (0.37 [9])
0.93 (0.88 [9])
–
33.1 ± 0.7 (35.8 ± 0.4 [9])
117.6 ± 2.4 (106 ± 1 [9])
Pdes (MPa)
–
0.15
0.31
0.61
1.1
28.8 ± 0.2
101.5 ± 0.7
Pdes (MPa)
–
0.082
0.17
0.33
–
27.9 ± 0.8
93.4 ± 2.6
3.2.3. Hydrogen absorption properties of the YMgCo2Ni2 compound
PCT desorption isotherms at different temperatures for YMgCo2Ni2–H2 system are shown in Fig. 7a. The feature of this interaction
is gradual slope of PH2CH curve in the range of the existence of YMgCo2Ni2Hx (x = 3.8 4.9 at room temperature). The obtained
values
of
heat
and
entropy
hydride
formation
(DH = 28.8 ± 0.2 kJ mol–1 H2 and DS = 117.6 ± 2.4 J mol1 H2 K1) are calculated from the corresponding van’t Hoff plot (Fig. 7b).
The absorption–desorption isotherms of hydrogen interaction
with YMgNi4, YMgCo2Ni2 and YMgCo4 at room temperature are
compared in Fig. 8. For YMgCo4 we observe inclined plateau with
significantly lower equilibrium pressure and lower hysteresis of
hydrogen absorption–desorption curves. At room temperature
the Pabs/Pdes hysteresis ratio in the system YMgCo4–H2 is 1.5,
whereas in YMgCo2Ni2–H2 it is 1.45, and 3.2 in YMgNi4–H2. Reducing the equilibrium pressure can be caused by an increase in the
size of the unit cell in the transition from Ni- to Co-containing
IMC, whereas a significant increase of the hydrogen absorption
capacity probably can be explained by electronic factors. PCT measurements showed complete reversibility of hydride formation in
the studied YMgT4-H2 systems. Single absorption/desorption plateau is observed in all three systems, which corresponds to formation/decomposition of one hydride phase only.
DS = 106 ± 1 J mol1 H2 K1 [9]. The latter correspond to the values of the equilibrium pressure one order lower than actually observed, and possibly caused by an error in the calculations of
thermodynamic parameters in [9].
3.2.2. Hydrogen absorption properties of the YMgCo4 compound
The investigation of hydrogen absorption properties of YMgCo4
compound was performed under the same conditions as that for
YMgNi4. It is interesting to note that hydrogen absorption capacity
of YMgCo4 reaches 6.8 at. H/f.u. (1.92 wt.%), which is higher by 70%
than the capacity of the isostructural Ni-containing compound
(Fig. 4).
Thermodynamic stability of the YMgCo4H6.8 hydride was determined from the equilibrium pressure of hydrogen desorption obtained at 293, 313 and 333 K temperatures. Desorption isotherms
and van’t Hoff plot for the YMgCo4–H2 system are shown in
Fig. 6. The values of heat and entropy hydride formation were calculated: DH = 27.9 ± 0.8 kJ mol1 H2 and DS = 93.4 ± 2.6 J mol1
H2 K1 (Table 2). For calculations of the thermodynamic parameters we assumed that there is only one hydride phase YMgCo4H6.8
in the system and have taken the values of the equilibrium pressures corresponding to the middle of desorption plateau
(3 H at./f.u.). However, it is possible also that the intermediate
hydride phase could be formed between solid solution of hydrogen
in IMC and saturated hydride YMgCo4H6.8. A faint plateau fracture
for 5 H at./f.u. content is possible evidence of this (Fig. 6a). The
3.2.4. Hydrogenation of the YMgCu4 compound
It was shown that in contrast to isostructural Co- and Ni-based
compounds YMgCu4 does not interact with hydrogen. We made
several unsuccessful attempts of hydrogenation after activation
in vacuum at temperatures of 623–693 K. The compound did not
absorb hydrogen at 2 MPa (293–623 K) and at 15 MPa H2 pressure
(293–473 K). This inertness to hydrogen of Cu-based compound
CH (wt.%)
0.0
0.5
1.0
1.5
10
10
(b)
(a)
333 K
313 K
293 K
1
2
2
PH (MPa)
PH (MPa)
1
0.1
0.1
0.01
0.01
0
1
2
3
4
CH (H at./f.u.)
5
6
7
3.0
3.2
3.4
-1
1000/T (K )
Fig. 6. PCT desorption isotherms (a) and van’t Hoff plot (b) for the YMgCo4–H2 system.
V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13
11
Fig. 7. PCT desorption isotherms (a) and van’t Hoff plot (b) for the YMgCo2Ni2–H2 system.
probably relates to the electron configuration of atomic Cu, namely
the full settlement of outer 3d electron shell. Recent calculations of
the electronic structure of the NdMgNi4H4 hydride showed that Hatoms form covalent bonds with 3d-orbitals of Ni-atoms [22].
Influence of d-element nature on hydrogen absorption properties
of RMgT4 compounds show up brightly in differences for Ni- and
Co-based compounds. In particular, hydrogen absorption capacity
of RMgCo4 under the same conditions is 70% higher than that for
RMgNi4. To explain these differences the additional quantumchemical calculations of the electronic structure and thermodynamic parameters are required.
3.3. Structural studies of saturated YMgT4Hx (T = Ni, Co) hydrides
3.3.1. Crystal structure of the YMgNi4H4 hydride
In spite of the large number of performed researches [9,15,23–
25], the crystal structure of YMgNi4H4 hydride has not been experimentally investigated due to its low thermodynamic stability. At
room temperature hydrogen desorption pressure is higher than
1 atm and the hydride decomposed before the structural studies
were made.
Theoretical calculations of the hydride structure were performed in [26,27]. Both results of quantum-chemical calculations
predicted that for YMgNi4H4 hydride the orthorhombic structure
(Pmn21) is more stable than the cubic structure of parent IMC (F43m). The lattice parameters of the rich hydride (a = 5.0033 Å,
b = 5.4296 Å, c = 7.2548 Å, V = 197.08 Å3) and the heat of its formation (37.7 kJ mol1 H2) were mentioned in the paper [26]. The
volume of orthorhombic unit cell (197.66 Å3) and heat of formation
(26.7 kJ mol–1 H2) were theoretically calculated in [27]. It is interesting to note that the heat of formation of the YMgNi4H4 hydride
(33.1 ± 0.7 kJ mol–1 H2), determined in this paper from the
desorption isotherms, are between theoretically calculated values
in [26] and [27]. The changes of cubic into orthorhombic structure
were observed upon formation of the NdMgNi4D3.6 [16] and
b-LaMgNi4D3.7 [28] hydrides, whereas the metal matrix of c-LaMgNi4D4.85 [28] and b-CeMgCo4D4.2 [19] preserves the cubic
structure of the parent IMC.
We performed in situ XRD studies to determine the YMgNi4H4
hydride structure using synchrotron radiation. Diffraction patterns
of YMgNi4 obtained by in situ SR XRD are shown in Fig. 9. Diffraction profile for YMgNi4H4 hydride was indexed in orthorhombic
unit cell (Pmn21) with parameters correlated with the initial cubic
p
p
structure of YMgNi4 as follows: aorth acub/ 2; borth acub/ 2;
corth acub; Vorth 1/2 Vcub. It is shown that the hydrogenation of
YMgNi4 is accompanied by the predicted orthorhombic deformation of the original cubic structure with volume expansion of
14.9%. Hydrogen sublattice of this hydride is probably isostructural
to NdMgNi4H4 [16], where hydrogen atoms occupy three types of
interstices: two types of trigonal bipyramids [Nd2MgNi2] (2a and
4b sites) and tetrahedra [NdNi3] (2a site).
At the first hydrogenation under 2 MPa pressure we failed to
achieve complete conversion of IMC into hydride (90% yield).
Therefore we increased the hydrogen pressure up to 4 MPa. and
heated the sample to 423 K. We observed the desorption of hydrogen and back conversion of orthorhombic hydride into a cubic IMC.
When the sample was cooled down to room temperature we got
almost single phase hydride due to hydrogen reabsorption. The results of refinement are given in Table 3. Our studies of the YMgNi4H4 crystal structure confirmed theoretical predictions [26,27]. In
particular, the experimentally observed volume of the unit cell
(198.23 Å3) coincides well with the value of 197.66 Å3 obtained
by quantum-chemical calculations [27]. No intermediate hydride
phase was observed in the process of formation and decomposition
of the YMgNi4H4 hydride.
3.3.2. Crystal structure of the YMgCo4H6.8 hydride
The structure of the YMgCo4H6.8 hydride was studied by in situ
SR XRD using the same method as that for the YMgNi4H4 hydride
(see ‘‘Experimental part’’). Diffraction patterns of initial sample
(293 K, in vacuum) and after complete hydrogenation (293 K,
1 MPa H2) are shown in Fig. 10. Unlike YMgNi4, the hydrogenation
of YMgCo4 compound does not lead to a distortion of the cubic
structure. It should be pointed out that upon hydrogenation we
did not observe the formation of any intermediate hydrides.
According to the absorption isotherm (see Fig. 6) the hydrogen
content at 293 K and 1 MPa H2 pressure is about 6.5 at. H/f.u. The
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V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13
Table 3
Crystallographic parameters of the YMgNi4H4 hydride obtained from in situ SR XRD
data (room temperature, 4 MPa H2). Space group Pmn21; a = 5.0292(4), b = 5.3996(4),
c = 7.3000(6) Å; V = 198.23(3) Å3; DV/V = 14.9%.
Atoms
Site
x
y
z
Biso (Å2)
Y
Mg
Ni1
Ni2
Ni3
2a
2a
2a
2a
4b
0
0
0
0
0.755(1)
0.298(1)
0.827(3)
0.4517(9)
0.987(1)
0.226(1)
-0.078(21)
0.140(21)
0.544(20)
0.528(21)
0.298(21)
0.91(8)
0.91(8)
0.45(4)
0.45(4)
0.45(4)
RBragg = 2.17%. Sample contains 1.1(1)% of the YMgNi4 parent phase (a = 7.021(1) Å).
(a)
Yobs
Ycalc
Yobs-Ycalc
Bragg position
(a)
Intensity (a.u.)
Fig. 8. PC dependences for YMgNi4, YMgCo2Ni2 and YMgCo4–H2 systems at room
temperature.
Yobs
Ycalc
Yobs-Ycalc
Bragg position
(b)
1.0 MPa H2
YMgNi4
Intensity (a.u.)
(b)
5
2 MPa H2
10
15
20
25
30
2θ (deg.)
YMgNi4 10%
YMgNi4H4
Fig. 10. In situ synchrotron X-ray diffraction patterns of the YMgCo4 alloy: (a)
original sample and (b) hydride at 1 MPa H2 pressure. Observed (Yobs), calculated
(Ycalc) and difference (YobsYcalc) diffraction profiles and Bragg’s peaks positions are
shown.
(c)
Table 4
Crystallographic parameters of the YMgCo4H6.8 hydride obtained from in situ SR XRD
data (293 K, 1 MPa H2). Space group F–43m; a = 7.5884(4) Å; V = 431.81(4) Å3; DV/
V = 22.7%.
4 MPa H2
YMgNi4 1%
YMgNi4H4
5
10
15
20
25
30
2θ (deg.)
Fig. 9. In situ SR XRD patterns of the YMgNi4 alloy: (a) parent sample, (b) hydride at
2 MPa H2 pressure and (c) hydride at 4 MPa pressure. Observed (Yobs), calculated
(Ycalc) and difference (Yobs–Ycalc) diffraction profiles and Bragg’s peaks positions are
shown.
formation of hydride leads to a cell volume increase of 23%. Specific
volume expansion for one hydrogen atom (3.1 Å3) is close to that
for YMgNi4H4 (3.2 Å3/at.H.). Crystallographic parameters of the
YMgCo4-based hydride are presented in Table 4.
Similar to the structure of CeMgCo4D4.2, we can assume that H
atoms occupy triangular faces MgCo2 (centres of R2MgCo2 bipyramids), 16e site [19]. Full occupancy of this position would give Hcontent of 6 at./f.u. From geometric considerations, another possible site for H occupation is Co4 tetrahedron, 4b site. This site is the
only position distance from which to the available H atoms exceeds
Atoms
Site
x
y
z
Biso (Å2)
R1
R2
Co
4a
4c
2a
0
1/4
0.6251(2)
0
1/4
x
0
1/4
x
2.1(1)
4.9(4)
1.08(5)
R1 = 0.87 Y + 0.13 Mg; R2 = 0.89 Mg + 0.11 Y; RBragg = 7.06%.
2.0 Å, a lower limit for H–H distances in metal hydrides. Complete
occupancy of 16e and 4b interstices corresponds to theoretical H
content of 7 at. H/f.u., which agrees well with maximum capacity
obtained from volumetric measurements, 6.8 at. H/f.u. Taking into
account that both 16e and 4b interstices are partially occupied in
cubic LaMgNi4D4.85 [28], such scenario of hydrogen occupancy
seems to be most plausible for YMgCo4-based hydride.
3.3.3. Crystal structure of the YMgCo2Ni2H4.9 hydride
The XRD pattern of YMgCo2Ni2H4.9 was measured by Bruker D8
diffractometer with Cu Ka radiation. Crystallographic parameters
of the YMgCo2Ni2H4.9 hydride are presented in Table 5 and the refined pattern in Fig. 11. The hydride has the same cubic structure
than YMgCo4H6.8 but with a smaller cell parameter and volume
V.V. Shtender et al. / Journal of Alloys and Compounds 603 (2014) 7–13
Table 5
Crystallographic parameters of the YMgCo2Ni2H4.9 hydride obtained at room
temperature. Space group F–43m; a = 7.3463(4) Å; V = 396.469(32) Å3; V/V = 14.4%.
Atoms
Site
x
y
z
Biso (Å2)
R1
R2
Co, Ni
4a
4c
2a
0
1/4
0.6203(2)
0
1/4
x
0
1/4
x
4.9(3)
4.7(7)
2.8(1)
R1 = 0.85 Y + 0.15 Mg; R2 = 0.92 Mg + 0.08 Y; RBragg = 11.9%.
13
and YMgNi4–H2 systems revealed that introduction of magnesium,
accompanied by shrinking of the unit cell, decreases the stability of
hydrides comparing to binary YCo2 and YNi2 compounds. Magnesium causes as well slight decrease in hydrogenation capacity.
The YMgCo4–H2 system is characterized by one absorption/desorption plateau. The values of heat and entropy of the YMgCo4H6.8 hydride formation were calculated: DH = 27.9 ± 0.8 kJ mol1 H2 and
DS = 93.4 ± 2.6 J mol1 H2 K1. The YMgCo2Ni2–H2 system
shows intermediate thermodynamic properties compared to the
ternary hydrides (DH = –28.8 ± 0.2 kJ mol–1 H2 and DS = 117.6 ±
2.4 J mol–1 H2 K–1). The formed YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compound in contrast
to the hydrides of isostructural YMgNi4 (as well as LaMgNi4 and
NdMgNi4 compounds), which undergo orthorhombic transformation around 4 H/f.u.
References
Fig. 11. X-ray diffraction pattern of YMgCo2Ni2 hydride. Observed (Yobs), calculated
(Ycalc), difference (Yobs–Ycalc) diffraction profiles and Bragg’s peaks positions for 1 –
YMgCo2Ni2 hydride (98 wt.%) and 2 – Y2O3 (2 wt.%) phases are shown.
as expected from the smaller H content. The formation of hydride
leads to increased volume by 14.4% near that observed for YMgNi4H4. Specific volume expansion for one hydrogen atom (3.1 Å3) is
similar to those observed for the two other hydrides.
A small occupation exchange between Y and Mg is observed on
the 4a and 4c sites, close to that observed for YMgCo4H6.8. It is very
interesting to observe that the Co for Ni substitution allow to avoid
the orthorhombic distortion observed for YMgNi4H4 hydride,
which should results from geometric constrains created by H insertion. The larger Co radius, increases the size of the interstitial sites,
and the H atoms have therefore enough space to be inserted whith
an isotropic cell expansion.
4. Conclusions
New ternary YMgCo4 and YMgCu4 and quaternary YMgCo2Ni2
compounds have been synthesized by mechanical alloying followed by further annealing. YMgCu4 does not interact with hydrogen under normal conditions. The hydrogenation capacity of
YMgCo4 reaches 6.8 at. H/f.u., which is substantially higher than
that for YMgNi4 (3.7 at. H/f.u.). This observation is very similar to
that for Ce-based compounds [19]. The PCT studies of YMgCo4–H2
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